The present disclosure generally relates to matrix acidizing of subterranean formations, and, more specifically, to methods for mitigating precipitation that may occur in conjunction with acidizing operations.
Treatment fluids can be used in a variety of subterranean treatment operations. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Unless otherwise specified, use of these terms does not imply any particular action by the treatment fluid or a component thereof. Illustrative treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control operations, and the like, which may include, for example, fracturing operations, gravel packing operations, acidizing operations, descaling operations, consolidation operations, and the like.
In acidizing operations, a subterranean formation containing an acid-soluble material (e.g., carbonates, silicates, or aluminosilicates) can be treated with an acid to dissolve at least a portion of the material. The acid-soluble material may exist naturally within the subterranean formation, or it may have been deliberately introduced into the subterranean formation in conjunction with performing a subterranean operation (e.g., proppant particulates or bridging agents). Dissolution of these acid-soluble materials can desirably open voids and conductive flow pathways in the formation that can improve the formation's permeability and enhance its rate of hydrocarbon production, for example. In a similar motif, acidization may sometimes be used to remove precipitation damage, occluding materials, mineral materials, perforation debris, and the like that can be present in the subterranean formation.
Carbonate formations often contain minerals that comprise a carbonate anion (e.g., calcite). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the formation matrix components. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic acid, formic acid, methanesulfonic acid, and the like) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include silicate and/or aluminosilicate minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to substances having the characteristics of silica, including silicates and/or aluminosilicates. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing material including carbonates (e.g., calcite), aluminosilicates, and other silicates.
Acidizing a siliceous formation (e.g., a sandstone formation or a clay-containing formation) or a formation containing a siliceous material is thought to be considerably different than acidizing a carbonate formation. Specifically, the treatment of a siliceous formation with the treatment fluids commonly used for acidizing a carbonate formation may have little to no effect, because mineral acids and organic acids do not effectively react with siliceous materials. In contrast to mineral acids and organic acids, hydrofluoric acid can react very readily with siliceous materials to produce soluble substances. Oftentimes, a mineral acid and/or an organic acid (e.g., formic acid, acetic acid, methanesulfonic acid, and the like) may be used in conjunction with a hydrofluoric acid-containing treatment fluid to maintain the treatment fluid in a low pH state as the hydrofluoric acid becomes spent. In some instances, the low pH of the treatment fluid may promote initial silicon dissolution and aid in maintaining the silicon in a dissolved state. At higher subterranean formation temperatures (e.g., above about 200° F.), it may be undesirable to lower the pH below about 1 due to mineral instability that can occur. Additionally, corrosion can be an inevitable problem that occurs when very low pH treatment fluids are used.
Although low pH treatment fluids may be desirable to aid in dissolution of siliceous materials, precipitation of insoluble fluorosilicates and aluminosilicates can still become problematic in the presence of certain metal ions. Specifically, dissolved siliceous materials can react with Group 1 metal ions (e.g., Na+ and K+) to produce insoluble fluorosilicates, fluoroaluminates, and aluminosilicates. The terms “Group 1 metal ions” and “alkali metal ions” are used synonymously herein. Other metal ions, including Group 2 metal ions (e.g., Ca2+ and Mg2+), may also be problematic in this regard.
The precipitation of insoluble fluorosilicates, fluoroaluminates, and aluminosilicates can block pore throats and undo the desirable permeability increase initially achieved by the acidizing operation. In some instances, the formation damage caused by the deposition of insoluble fluorosilicates, fluoroaluminates, and aluminosilicates can be more problematic than if the acidizing operation had not been conducted in the first place. In contrast to many metal ions, ammonium ions (NH4+) are not believed to promote the formation of insoluble fluorosilicates, fluoroaluminates, and aluminosilicates. Accordingly, treatment fluids comprising an ammonium salt are frequently used in conjunction with acidizing operations conducted in the presence of siliceous materials, as discussed further below. However, use of ammonium salts in treatment fluids can considerably increase the cost of performing a treatment operation.
Problematic alkali metal ions or other metal ions can come from any source including, for example, the treatment fluid, a component of the treatment fluid, or the subterranean formation itself. For example, the carrier fluid of a treatment fluid may contain some sodium or potassium ions unless costly measures (e.g., deionization) are taken to limit their presence. Alkali metal ions, in particular, are widely distributed in the environment and can be especially difficult to avoid completely when conducting a subterranean treatment operation. As discussed further below, a variety of strategies have been developed to address the most common sources of problematic metal ions encountered when conducting subterranean treatment operations. As a general rule, it has been conventional to minimize the effective concentration of metal ions during treatment so to avoid the foregoing issues and others.
One strategy that has been used with some success to avoid the damaging effects of metal ions may include introducing a sequence of pre-flush treatment fluids into the subterranean formation prior to performing an acidizing operation with hydrofluoric acid. For example, a pre-flush treatment fluid comprising a mineral acid or an organic acid can be used to dissolve acid-soluble formation matrix components and remove at least a portion of the problematic metal ions from the formation. Thereafter, another pre-flush treatment fluid comprising an ammonium salt can be introduced into the subterranean formation to displace the remaining formation matrix metal ions and leave the formation enriched in ammonium ions. Although this approach can be used successfully for mitigating unwanted precipitation, it can considerably add to the time and expense needed to perform an acidizing operation.
Another strategy that can be used to mitigate the effects of metal ions in acidizing operations is to introduce a chelating agent into the subterranean formation. Although this strategy can be successfully used for Group 2 metal ions, transition metal ions, and aluminum, for example, chelation is believed to be less effective for alkali metal ions. In addition, aluminum ions, for example, in the presence of a chelating agent and hydrofluoric acid may form fluoroaluminate complexes, such as LAIF2 and LAIF (L=chelating agent) or higher fluoroaluminates, whose overall charge is determined by the charge of the chelating agent. However, these complexes may still precipitate in the presence of alkali metal ions. Further, many chelating agents are utilized in their salt form, which is many times their Na+ or K+ salt form. Thus, use of a chelating agent, although mitigating precipitation effects from certain metal ions, can actually exacerbate the precipitation effects of alkali metal ions due to the metal ions being introduced from the chelating agent. Sometimes the free acid or ammonium salt forms of chelating agents can be used to avoid this issue, at least in principle, but the free acid and/or ammonium salt forms of many chelating agents are either unknown or not commercially available at a reasonable cost. Furthermore, many common chelating agents are not biodegradable or present other toxicity concerns that can make their use in subterranean formations problematic.